The present invention provides a positive lithographic printing plate and describes a method for the laser exposure of photographic, so called, "direct-to-plate" lithographic printing plate materials by a laser imagesetting device. In particular, the invention relates to laser exposure of photo-lithographic "direct-to-plate" substrates which employ the diffusion transfer reversal process, hereinafter called DTR, and more specifically to photo-lithographic DTR elements suitable for exposure by gas and solid state laser wavelengths. Materials suitable for exposure by lasers emitting in the red region of the electromagnetic spectrum, e.g. helium-neon (HeNe) or some semiconductor diode lasers, are disclosed, e.g. in U.S. Pat. No. 5,059,508 and U.S. Pat. No. 4,501,811 and at least one such material is commercially available and marketed by AGFA-GEVAERT N.V. under the trademark SETPRINT HeNe-plate PP410. An example of a photo-lithographic DTR material suitable for laser exposure in the infra-red region of the electromagnetic spectrum, and therefore capable of being exposed by solid state lasers emitting at infra-red wavelengths, was announced by XENOTRON Co. in "Ifra Newspaper Techniques" No 7 p27, as early as Oct. 1985.
The diffusion transfer reversal process, as described e.g. in U.S. Pat. No. 2,352,014, is a photo-chemical process which through the steps of, imagewise exposure to electromagnetic radiation and chemical processing, results in the formation of a silver image in all regions which were exposed to electromagnetic radiation and a DTR-silver image in all regions which were not exposed to electromagnetic radiation.
In photo-lithographic "direct-to-plate" materials, the silver image and DTR-silver image have the particular characteristics that after chemical processing, the saver image forms water receptive ink repellent regions and the DTR-silver image forms water repellent ink receptive regions. Thus the DTR-silver image, which is formed in areas which were not exposed to electrornagnetic radiation and which is ink receiving, is the image which is printed when the DTR "direct-to-plate" material is mounted on an offset press, inked and the inked image is transferred to a printing material. The lithographic DTR printing plate is thus a "positive plate" as defined in "Chemistry for the Graphic Ans" by P. J. Hartsuch, --Graphic Arts Technical Foundation--Pinsburgh, PA (1979), in that a "direct-to-plate" DTR photo-lithographic recording substrate must be exposed in all areas for which it is desired that the printing plate formed after chemical processing be ink repellent. The term "positive plate" is used herein in accordance with this definition.
According to well known electronic prepress embodiments, images to be printed by offset printing means are assembled and edited electronically on a page layout workstation or "front end", and then transmitted to a raster image processor or, "RIP" for digital color separation, half-tone screening and image rasterization. The "RIP image", that is the rasterized image to be printed by offset printing means, is then transmitted from the RIP to a laser imagesetter for photographic or film recording. Such an electronic prepress system was described as early as 1977 in U.S. Pat. No. 4,004,079 which also teaches that electronic images can be recorded directly onto printing plate materials.
Electronic prepress systems and components which perform these steps are commercially available and marketed e.g. by MILES AGFA DIV. under the trademark COLORSCAPE, which is a complete electronic prepress system, or e.g marketed by EASTMAN KODAK under the trademark PROPHECY, which is a front end, or e.g. marketed by HYPHEN LTD. under the trademark RIPWARE which is a RIP, or for example marketed by LINOTYPE-HELL under the trademark LINOTRONIC L300 or LINOTRONIC L600, which are imagesetters.
Furthermore, it is also well known in the prior an that to create a "positive plate" by electronic prepress means, a positive image is displayed and edited at the front end and transmitted to the RIP, a positive RIP image is then formed by the RIP and transmitted to an imagesetter and a "positive film" is then recorded by the imagesetter on a DTR photographic substrate which is not lithographic. Here the imagesetter provides imagewise exposure to laser radiation in all image regions which are intended to be inked by the press. After chemical processing a silver image is formed in these imagewise exposed regions which are intended to be inked at the press. The silver image of a positive film has the particular characteristic that it is opaque to certain electromagnetic wavelengths. The DTR silver image formed in the non-exposed background regions of a positive film and which is not intended to be inked by the press, has a uniform background density which is transmissive to these same electromagnetic wavelengths.
It is customary for imagesetter manufacturers to specify a DTR recording substrate which is photographic but not lithographic. Such a substrate is commercially available and marketed by AGFA GEVAERT N.V under the trademark, ZHN. These non-lithographic DTR materials are used in imagesetter devices to produce positive films.
In prior art techniques, these positive films are used as a master for a further exposure step whereby a photo-lithographic DTR substrate is exposed while in contact with an imagesetter positive film and after chemical processing this photo-lithographic substrate provides a positive printing plate. The details of image formation and the inking properties of these prior an photo-lithographic DTR materials are similar to those described above for "direct-to-plate" DTR materials. The prior an materials used for plate making by contact exposure include DTR photo-lithographic printing plate materials which are sensitive to exposure by a tungsten light source. Examples of DTR photo-lithographic plate materials used for contact exposure by a tungsten lamp are commercially available and marketed by MITSUBITSHI under the trademark SILVERMASTER and by AGFA GEVAERT N. V., under the trademark SUPERMASTER.
The additional exposure step of prior an platemaking techniques provides an image reversal in that as a result of placing an exposed and processed imagesetter positive film in contact with an unexposed DTR photo-lithographic plate material and exposing the plate material by a tungsten lamp, tungsten light will only reach the regions of the plate material which are in contact with the transmissive regions of the positive film. The transmissive regions of the positive film are the background regions of the printed image which are not intended to be inked at the press. The prior art, double reversal method which produces a positive printing plate thus requires that the imagesetter expose all ink receiving areas and that the plate material be exposed in all ink repellent areas.
Additional intermediate steps in lithographic platemaking by conventional means, particularly for large presses, involve the mechanical assembly or so called "stripping" of multiple positive rams onto a "fiat". A fiat is assembled for each ink to be used at the press and each flat includes at least one positive film assembled onto a background region. The background region may include such features as registration apertures, crop marks, border regions or color calibration strips used to monitor press conditions during printing runs. In the particular case where a positive printing plate is desired, a background material is used which is transmissive to tungsten lamp radiation. These positive fiats, at least one for each ink used at the press, which each include one or more positive films are assembled and contact printed onto photo-lithographic materials as described above. Each positive fiat results in a positive lithographic printing plate which is then mounted on the press and printed, allowing the press to print multiple positive films in a single pass.
In stripping operations, the size and shape of the flat is determined by the particular press being used for printing. Features such as the final plate overall dimensions and the dimensions of the plate mechanical gripper areas, used for mounting the plate on the press, must match the particular press cylinder dimensions for proper press mounting. The stripping operation is labor intensive requiring many assembly and masking steps and a high skill level is required since precise alignment of positive films and press mounting features are critical for acceptable printing in color work.
Recent trends in platemaking have provided a means for mounting a DTR photo-lithographic "direct-to-plate" material directly into a laser imagesetter and exposing a plate with the imagesetter. These materials, as described above, are available in web form and can be used to fabricate plates for medium length printing runs with as many as 10,000 printed images being produced from a single plate. Since in this direct-to-plate exposure method, the prior an step of exposing the plate material while in contact with a previously exposed imagesetter positive film is eliminated, so too is the image reversal which occurs during the plate exposure step. Thus direct-to-plate expose of DTR materials requires that the imagesetter expose only the ink repellent background regions of the RIP image and not the ink receiving image which is intended to be printed at the press. The imagesetter must expose a negative of the RIP image and it must expose all those regions of the final plate for which it is desired that the plate be ink repellent.
Conventional laser imagesetting devices, such as those imagesetters commercially available and marketed by MILES Inc. AGFA Div. under the trademark SELECTSET 5000 and SELECTSET 7000 as well as other imagesetter devices marketed by LINOTYPE-HELL A.G. under the trademark LINOTRONIC L300 and LINOTRONIC L600, are capable of laser exposure of "direct-to-plate" DTR photo-lithographic materials and are capable of exposing a negative of the positive RIP image. It is a particular problem however that imagesetters are not capable of exposing direct-to-plate recording substrates over all regions for which it is desired that the final plate be ink repellent. Since unexposed regions of "direct-to-plate" materials are ink receptive at the final plate, steps must be taken after chemical processing to remove unwanted ink receptive regions of plates exposed by prior art imagesetters. These unwanted ink receptive regions are removed either by chemical or mechanical means, e.g. by cutting the undesired ink receptive regions from the plate before mounting it on the press.
It is a specific problem of conventional laser imagesetters, that in regions where the recording substrate is mechanically clamped during exposure that laser exposure is not possible in the regions shielded by the mechanical clamp mechanism. The inability to expose direct-to-press materials in clamp regions is well known for both imagesetters which expose web mounted recording materials as well as imagesetters which expose sheet materials. In addition to the inability to expose clamped material, unexposed regions of "direct-to-plate" materials also remain in the regions surrounding the RIP image. This problem has been solved in the past by the creation of a very large RIP image at the front end such that the RIP image is at least the same size as the plate to be produced. Here the RIP image includes the image to be printed plus a background region which is to be ink repellent at the press. This solution to the problem requires that the front end operator know the plate size prior to sending the image to the RIP. This solution is not always convenient since often times the front end images may be sent to a service bureau for off sight imagesetting and printing. In addition, if a large fiat is to be assembled from more than one RIP image, then this too would have to be determined by the front end operator.
Having outlined the prior art methods of direct-to-plate exposure using a laser imagesetter and electronic prepress means and having detailed the steps of stripping multiple positive films into a fiat and having described certain problems in the prior art, namely the need to remove unexposed regions of a direct-to-plate material before mounting the plate onto the press, it is accordingly a general object of the present invention to provide a "direct-to-press" positive lithographic printing plate comprising at least one ink receiving RIP image region and at least one ink repellent background region which is not included i-n the RIP image.
It is a specific object of the present invention to provide a "direct-to-press" imagesetting exposure method which allows laser exposure of "direct-to-plate" materials in regions normally shielded by mechanical clamps and in regions not included in the RIP image.
It is a further object of the present invention to provide a laser imagesetter exposure method which allows exposure of a "direct-to-plate" recording substrate in all regions for which it is desired that the final plate be ink repellent thus eliminating the need for stripping, mechanical cutting, chemical removal, or additional exposure steps to remove unwanted ink receptive regions of the exposed "direct-to-plate material".
It is another object of the present invention that the parameters of the "direct-to-press" exposure mode of the present invention be under the control of the imagesetter operator requiring no interaction with the RIP or the front end and that the improved exposure method include only changes to the imagesetter.